1. Field of the Invention
The present invention relates generally to an improved automatically retractable utility knife and more particularly to an improved automatically retractable utility knife having a modified blade carrier guide slot that decreases forward pressure on a user""s finger during extension of the utility blade.
2. Description of the Related Art
Automatically retractable utility knives, also known as safety knives, are well known in the art, including, for example, commonly owned U.S. Pat. Nos. 6,058,607 and 5,813,121, the contents of which are incorporated by reference herein in their entirety. Automatically retractable utility knives in a rested position feature a blade in a retracted position in a housing. Continuous application of pressure by a user on a thumb piece is required for the blade to remain in the projected position. A spring attached between the housing and a blade carrier automatically retracts the blade upon release of the thumb piece. The spring tension is such that when the blade is projected and inserted in a material being cut, e.g., cardboard, the friction between the blade and the material is generally sufficient to retain the blade in the projected position.
A drawback of this type of utility knife is that it requires constant pressure applied to the thumb piece to cause the blade to remain in the extended position before the cut. Since one object of automatically retractable knives is to retract the blade as quickly as possible to avoid injuring a user, high tension pull springs are frequently used. These springs require even greater continuous pressure on the thumb piece in order for the blade to remain in an exposed position prior to cutting. Through continued use, the user may become fatigued, which may lead to injury.
Accordingly, it would be desirable to have an automatically retractable utility knife with a partial locking mechanism that assists a user in retaining the blade in an extended position but that still permits automatic retraction of the blade.
The present invention is an improved automatically retractable cutting device having (a) a housing comprising a guide slot, (b) a return spring within the housing, and (c) a blade carrier disposed within the housing and biased by the return spring. The blade carrier includes a thumb piece extending through the guide slot for facilitating movement of the blade carrier in the housing to extend and retract a utility blade. The housing includes at least one notch disposed in the guide slot, and the thumb piece includes a protuberance adapted to at least partially engage the notch during movement of the thumb piece along the guide slot to enable the thumb piece to be held in position adjacent to the notch with reduced force. The protuberance is adapted to frictionally engage the at least one notch. The notch may include a forward direction ramping surface, whereby reduced force is required to prevent further blade extension, and/or a reverse direction ramping surface, whereby reduced force is required to prevent blade retraction.
An alternative improved cutting device includes (a) a housing having a guide slot; and (b) an automatically retractable blade carrier disposed within the housing. The blade carrier comprises a thumb piece extending through the guide slot for facilitating movement of the blade carrier in the housing to extend and retract a utility blade. The guide slot includes one or more notches, each having at least one surface ramped at an angle of less than ninety degrees. The thumb piece is adapted to frictionally engage the at least one notch, preferably by means of a protuberance on the thumb piece.
A further alternative improved cutting device includes a housing having a guide slot, a blade carrier having a thumb piece extending through the guide slot for facilitating movement of the blade carrier in the housing to extend and retract a utility blade, and a return spring within the housing for biasing the blade carrier. A plurality of notches is disposed along a length of the guide slot, and the thumb piece is adapted to frictionally engage each of the plurality of notches in order to decrease user force to maintain extension of the utility blade.
A cutting device according to the invention includes a housing having an upper portion and a lower portion adapted for assembly, a return spring within the housing, and a blade carrier disposed within the housing. The blade carrier is biased by the return spring and includes a thumb piece for facilitating movement of the blade carrier in the housing to extend and retract a utility blade. The housing includes a guide slot formed by the upper portion and lower portion. That portion of the guide slot on the upper portion includes a first set of notches, and that portion of the guide slot on the lower portion includes a second set of notches, with the first and second set of notches being substantially symmetrically aligned when the upper and lower portions of the housing are assembled. The thumb piece frictionally engages at least one of the first and second sets of notches during extension of the blade. If desired, a protuberance may be included on the thumb piece to at least partially engage each notch during movement of the thumb piece.